Planet Nine from outer space

planet nine
Hypothetical orbit (orange) of a hypothetical Planet Nine. Credit: Caltech & R. Hurt (IPAC).

By now you’ve probably heard of Planet Nine, the hypothesized super-Earth lurking in our solar system well past Pluto. But what you may not have heard is where the idea came from in the first place, and which teams are out there actively searching for such a world. For my latest Nature feature I tagged along on an observing run in Hawaii with Scott Sheppard and Chad Trujillo, who use some of the world’s largest telescopes to hunt for distant solar system objects. It was their 2012 discovery of a faraway world in the Kuiper belt — the part of the solar system that includes Pluto — that set off the current race for Planet Nine. Sheppard and Trujillo’s work shows that some Kuiper belt objects travel in extreme orbits that might — just might — have been shaped by the gravitational influence of an unseen super-Earth. Will we ever see a Planet Nine? Odds are against it. But we are sure to uncover weird and wonderful smaller worlds out there.